COMMON MINERALS in IGNEOUS ROCKS Part 1: Minerals in Granitic Rocks

GEOLOGY 17.01: Mineralogy

LAB 5: COMMON MINERALS IN IGNEOUS ROCKS Part 1: Minerals in Granitic Rocks

Learning Objectives: • Students will be able to identify the most common minerals in granitoids • Students will be able to identify the most common accessory minerals in granitoids • Students will be able to classify granitoid rocks based on mineral content

New Minerals : Muscovite*, Biotite*, Hornblende*, Tourmaline*, Apatite*, Rutile*

Review Minerals : Quartz, Plagioclase, Microcline

MINERALS IN GRANITIC ROCKS Granitic rocks (felsic to intermediate intrusive rocks) are composed mainly of the three minerals quartz, plagioclase, and potassium feldspar and their classification is based upon the relative proportions of these three minerals that you examined in previous labs. Additional major minerals (>5% of the rock) in granitic rocks may include micas (muscovite and biotite) and the amphibole hornblende. Common accessory minerals (<2% of the rock) include apatite (Ca 5[PO 4]3[OH,F,Cl]), tourmaline (Na[Al,Fe,Li,Mg,Mn]3Al 6[Si 6O18 ][BO3][OH,F]4), and rutile (TiO 2).

Micas are a group of hydrated aluminosilicate minerals. They are sheet silicates and exhibit perfect basal cleavage. Although they are monoclinic minerals, micas commonly form hexagon-shaped crystals. Muscovite (KAl 2Si 3AlO 10 [OH,F] 2) and 2+ biotite (K[Mg,Fe ]3AlSi 3O10 [OH,F] 2) are the two most common micas.

Examine the hand samples MUSCOVITE and BIOTITE, and document their physical properties.

Examine thin-section of MUSCOVITE and BIOTITE and document the optical properties of these minerals.

Amphiboles are a diverse group of rock-forming inosilicate minerals that are composed of double chain SiO 4 tetrahedra. Amphiboles are hydrous minerals, contain Fe and/or Mg, and most amphiboles contain Ca, Na or K as well. Amphiboles have two cleavage distinct planes at 124°/56°. Hornblende (Ca 2[Mg,Fe,Al] 5[Al,Si] 8O22 [OH] 2) is the most common amphibole, and is a common constituent of both igneous and high-grade metamorphic rocks.

Examine the hand sample HORNBLENDE, and document this mineral’s physical properties.

Examine the thin-section HORNBLENDE and document this mineral’s optical properties.

Felsic igneous rocks commonly contain small amounts of minerals that accommodate the ions from the magma that do not fit into the structure of the common silicate minerals. For example, titanium is commonly incorporated in to rutile (TiO 2), phosphorus into apatite (Ca 5[PO 4]3[OH,F,Cl]), and boron into tourmaline (Na(Al,Fe,Li,Mg,Mn) 3Al 6(Si 6O18 )(BO 3)3(OH,F) 4).

Rutile is a high relief, golden-brown colored mineral that typically forms small prismatic or needle-like crystals. Apatite is a colorless, uniaxial mineral with low interference colors, and so may be confused with quartz. However, apatite commonly displays well-developed hexagonal cross-sections, has a negative optic sign, and higher relief than quartz. Tourmaline occurs as prismatic crystals that commonly have a roughly triangular cross-section. One distinctive optical property of tourmaline is that it is pleochroic and displays its strongest colors when oriented north-south on the stage.

Examine the hand samples APATITE and TOURMALINE and document their physical properties.

Examine thin-sections RUTILE, APATITE, and TOURMALINE and document the optical properties of these minerals.

CLASSIFYING QUARTZ-BEARING PLUTONIC ROCKS (GRANITOIDS) The Streckheisen classification scheme for igneous rocks is used commonly and is based upon the relative proportion of three components: quartz, K-feldspar, and plagioclase. The major additional mineral that is present (muscovite, biotite, hornblende, etc) are used as modifiers to the rock name (e.g., biotite granite, hornblende syenite).

Review the optical properties of quartz, K-feldspar and plagioclase. Examine samples GRANITOID 1 and GRANITOID 2, and estimate the percentage of these minerals in each sample. Use the figure to the right to help you to estimate the percentage of each of these three mineral components. Recast these percentages relative to the total of these three components. Use this information to classify the sandstone using the Streckheisen classification diagram.

Streckheisen Diagram for Classification of Quartz-Bearing Intrusive Rocks

GRANITOID 1

% Quartz: ______% K-Feldspar: ______% Plagioclase: ______

Total = (Quartz + K-Feldspar + Plagioclase) = ______

% Quartz/Total = ______

% K-Feldspar/Total = ______

% Plagioclase/Total = ______

Other Major Minerals Present: ______

Accessory Minerals Present: ______

Rock Name: ______

GRANITOID 2